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February Geology and GSA Today media highlights

02.02.2005

The February issue of GEOLOGY covers a wide variety of potentially newsworthy subjects. Topics include: earthquakes in the central Indian Ocean and possible break-up of the Indo-Australian tectonic plate; dynamics of the Chicxulub impact tsunami; sea-level rise and the future of reef islands; evidence for abrupt climate change triggered by meltwater from glacial Lake Iroquois; new evidence from the Late Ordovician of CO2 as driver of climate change; and new support for a causal relationship between changes in Earth’s orbit and the end of Earth’s penultimate ice age.

Highlights are provided below. Representatives of the media may obtain complimentary copies of articles by contacting Ann Cairns at acairns@geosociety.org. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Ann Cairns for additional information or other assistance.

Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org

Geology

Basinward transport of Chicxulub ejecta by tsunami-induced backflow, La Popa basin, northeastern Mexico, and its implications for distribution of impact-related deposits flanking the Gulf of Mexico

The impact of a large extraterrestrial object on the Yucatan platform 65 million years ago created one or more large tsunami, or huge waves, that displaced a large volume of the Gulf of Mexico onto the adjoining continent. This water, rushing back toward the Gulf, would have possessed tremendous erosive capacity and the ability to carry sediment and fossils from the coast into deep water. Geologists working in northeastern Mexico recently discovered evidence for this southward backrush of water in the deposits of ancient valleys scoured into the continental shelf. The valley-fill deposits, laid down at the end of the Cretaceous period, contain abundant grains of formerly molten rock (ejecta) thrown from the Chixculub impact crater. These grains evidently arrived in northeastern Mexico before the tsunami, and the backwash of Gulf-bound water eroded them from coastal regions and concentrated them in the valleys as flow velocity slowed adjacent to the temporarily lowered sea level of the Gulf. Also present in the valley-fill deposits are fossils eroded from coastal settings; these organisms provide firm evidence for the source of the flow. Although the duration of the backflow is unknown, it is likely to have continued for hours or days after the initial impact. These deposits therefore provide evidence for an enormous sediment-recycling system set up by the run-up of tsunami waves onto coastal North America shortly after the Yucatan impact. This discovery confirms the grand scale of sediment and fossil recycling at the end of the Cretaceous and helps to explain why the latest Cretaceous fossils are commonly found above the impact rocks in the Gulf of Mexico region.

Catastrophic meltwater discharge down the Hudson Valley: A potential trigger for the intra-Allerød cold period

Given the possibility of future abrupt climate change driven by anthropogenic greenhouse gas emissions much effort has gone into understanding potential triggers and climatic feedback mechanisms for rapid climate fluctuations. Of particular interest to researchers in the last couple of decades has been the role of alterations of oceanic thermohaline circulation in driving abrupt climate-change events. Glacial freshwater discharge to the Atlantic Ocean has often been postulated to drive climatic fluctuations during deglaciation because it may inhibit oceanic thermohaline circulation. However, directly linking meltwater-discharge events with individual climate oscillations has been difficult because of challenges in determining the location, timing, and amount of meltwater discharge. Here we present both onshore and offshore evidence that establishes the timing of the catastrophic draining of Glacial Lake Iroquois (in the Lake Ontario Basin) to the North Atlantic via the Hudson River Valley. Approximately 3.5 x 1012 m3 of freshwater was discharged to the North Atlantic ~13,350 yr B.P. during this event. This meltwater release triggered a reduction in thermohaline circulation causing less heat to be transported to the North Atlantic and resulting in an abrupt cooling of the circum–North Atlantic region for ~200–300 years (the Intra-AllerØd Cold Period).

Bending as a mechanism for triggering off-axis volcanism on the East Pacific Rise

Most volcanic activity at the boundary of two rapidly diverging tectonic plates is focused in a narrow region along the caldera or summit of the spreading center axis, but some fraction of the eruptions occur up to 20 km away from the axis on the ridge flanks. Reves-Sohn and Sims propose that gravitational bending stresses play a key role in triggering and localizing off-axis volcanism at fast-spreading centers while simultaneously building abyssal hill fabric into young oceanic crust. The authors use analytical models to demonstrate the effectiveness of bending cracks for tapping lower crustal melt reservoirs, and they also develop a conceptual model linking upper mantle and lower crustal melt migration to lithospheric stress patterns and off-axis volcanism.

The coiled shells of ammonoid cephalopods are some of the most popular marine invertebrate fossils. Ammonoids are also well-known for their rapid rates of origination and extinction. This research paper presents results of an analysis of a new database of originations and extinctions for ammonoids from the Jurassic and Cretaceous periods (65–208 Ma). For one main ammonoid group, the Ammonitina, origination and extinction patterns show a white noise signal. White noise is characterized by a lack of long-term memory, such that an event at one time is uncorrelated with previous events. For example, originations of new ammonite taxa during one time interval apparently do not provide opportunities to fuel the appearance of additional new taxa in subsequent intervals. Rather, processes acting on short timescales of less than a few million years were the primary controls on ammonite origination and extinction.

Long-lived glaciation in the Late Ordovician?: Isotopic and sequence-stratigraphic evidence from western Laurentia

Variations in atmospheric CO2 are commonly taken to be the main driver of climate change on geological timescales. However, there is considerable uncertainty and debate about this fundamental issue. In part this issue remains unresolved because of the apparent exceptions to the link that have been noted in the rock record. An important one is the troubling mismatch between climate and apparent atmospheric CO2 levels during the Late Ordovician (ca. 440 Ma) glaciation of Gondwana that has been noted by many authors. The Late Ordovician glaciation has been difficult to reconcile with the evidence from models and proxy data that all converge on high levels of atmospheric pCO2 for the early Paleozoic at ~ 14–16 times preindustrial atmospheric levels. Our isotopic and sequence stratigraphic results support a different model for changes in carbon dioxide that confirm its role as the primary climate driver. We suggest an episode of pCO2 drawdown cooled Earth and initiated ice sheet growth some ten million years earlier than previously recognized. Subsequently, CO2 levels rose as the ice sheets reached their maximum extent and covered exposed silicate rock. The changes in weathering and rising CO2 caused the ice to quickly melt.

Earthquakes in the central Indian Ocean testify to unusually elevated stress levels in this region and provide a key indicator of the way the forces that drive plate motion are transmitted to the interior of large tectonic plates. We use the earthquake characteristics to model stress distribution in the Indo-Australian plate and show a low degree of stress coupling between the surface plate and the subducting slabs descending beneath Indonesia.

Controversy surrounds the relict glacier ice in Antartica’s Beacon Valley, purported to be of Miocene vintage because overlying debris contains undisturbed volcanic ash dated at 8.1 Ma. Ng et al. have now re-examined cosmogenic 3He data for the debris, which formed as a sublimation lag of the dirty ice beneath. They showed that as recently as 310 Ka the debris layer was much too thin to hold the deep ash wedges seen today. This finding casts doubt on the stratigraphic interpretation of old ice based on the ash, but reveals a new evolutionary picture of the debris cover as it became spectacularly patterned by contraction-crack polygons driven by seasonal temperature changes.

Andrew Y. Glikson, Australian National University, Research School of Earth Sciences, Canberra, Australia. Pages 125-128.

Until recently the nature of the early Earth crust was known mainly from studies of small continental relics dominated by granite, whereas the nature of the other ~90% of Earth crust remained unknown, except by indirect methods. Glikson’s paper examines the geochemical evidence provided by ejecta fallout of large asteroid and comet impacts on the early Earth (2.4–3.5 Ga), as preserved in sediments, which indicate that the earth crust consisted largely of oceanic-type basaltic rocks, small continental nuclei, and large transient impact basins (similar to the lunar Maria). This model of early Earth is consistent with considerations based on chemical and isotopic evidence from early volcanic and plutonic terrains.

Through much of the Pliocene (2.5–4.9 Ma) Antarctic sea-ice concentration was lower than today. The low sea-ice concentration is evident from the proportion of different algal fossils preserved in marine sediments. The results indicate that reduced winter sea-ice concentrations persisted through much of the Pliocene and at times were 78% and 61% relatively less concentrated than today. This was calibrated from the modern relationship between sea-ice concentration and different colonial forms of the algae Eucampia antarctica.

Timing of Early Cretaceous angiosperm diversification and possible links to major paleoenvironmental change

The timing of the early radiation of flowering plants (angiosperms) during the Early Cretaceous is still a matter of debate. Pollen results from well-dated marine sedimentary successions (West Portugal) provide evidence for a stepwise increase in angiosperm diversity and abundance during the late Early Cretaceous (ca. 124–104 Ma). Comparison of the angiosperm pollen results with data on global palaeoenvironmental change suggests a link between the rapid evolution of flowering plants and major climatic and oceanographic perturbations during this time interval.

New model of reef-island evolution: Maldives, Indian Ocean

P.S. Kench, The University of Auckland, School of Geography and Environmental Science, Auckland, NA 1001, New Zealand, et al. Pages 145-148.

A new model of reef island evolution in the Maldives challenges popular perception of island susceptibility to erosion with future sea-level rise. Drilling and radiocarbon dating evidence shows reef islands in the Maldives formed 5,000 years ago on submerged reefs. Following island development coral reefs continued to grow upward around islands reducing water depth and wave energy that affects islands. Significantly, results show that sea-level rise will increase water depth and wave action on reefs but to levels below those experienced by islands during their formation. Consequently reef islands are resilient landforms that will be unaffected by future sea-level rise.

Late Quaternary intensified monsoon phases control landscape evolution in the northwest Himalaya

Large landslide deposits in steep mountainous terrains are spectacular mass-movement features, but they also remind us of the natural hazards often encountered in the high mountain belts of the world. In the northwestern Himalaya, India, thirteen landslide deposits up to a kilometer long and half a kilometer thick are the vestige of catastrophic mass movements in the past. The landslides dammed rivers and formed lakes, which existed for several millennia. Today, mass movements also occur during years with anomalously high monsoonal precipitation that reaches dry regions in the higher sectors of the Himalayan range, although large landslides are not common. The lake deposits behind the former landslide dams contain organic material that can be dated using the radiocarbon method. Two temporal clusters at about 27,000 and 8000 years ago were identified and correspond to episodes when monsoons were much stronger, protracted, and resulted in increased rainfall reaching regions, which are dry today. The triggering of the landslides can be explained with sustained humid conditions that gave rise to increased pore pressures and enhanced fluvial erosion by lateral scouring of rivers at the foot of the steep hillslopes.

Manifestations of hydrothermal discharge from young abyssal hills on the fast-spreading East Pacific Rise flank

Most cooling of ocean lithosphere by hydrothermal fluid circulation occurs in a vast, abyssal hill-dominated terrain on the flanks of the mid-ocean ridge, yet few sites of hydrothermal venting have been located in this largely unexplored region. We describe newly discovered geological and biological manifestations of hydrothermal activity at two sites on young abyssal hills flanking the East Pacific Rise. Features observed at these sites include: hydrothermal deposits, hyperthermophilic microbes, and sediment structures possibly formed from fluid expulsion. To explain these features, we suggest that abyssal hill hydrothermal venting occurs in frequent bursts, possibly triggered by earthquakes. Such widespread and oft-repeated pulses of hydrothermal venting may stimulate microbial blooms on abyssal hill fault scarps, thus providing a potential food source for ridge flank biota and an opportunity for researchers to sample the subseafloor biosphere.

Structure of the penultimate deglaciation along the California margin and implications for Milankovitch theory

While the detailed climate history of the warming after the last ice age, known as the last deglaciation (~20,000–10,000 years ago), is well known, little is known about the previous or penultimate deglaciation (~130,000–120,000 years ago). This is because the quickly accumulating marine sediments that contain the most detailed records of climate during this interval of time are now deeply buried and hard to retrieve and because the most frequently used sediment-dating method (carbon-14) does not work this far back in time. Although these problems make studying the penultimate deglaciation difficult, determining whether rapid climate changes occurred during this deglaciation (similar to those that occurred during the last deglaciation) is critical for predicting future climate change. Furthermore, several recent studies of the penultimate deglaciation have suggested that it occurred earlier than previously thought calling into question the prevailing theory that changes in Earth’s orbit are responsible for the ice ages. New results from a marine sediment core drilled in Santa Barbara Basin, offshore Southern California, and several other cores drilled along the Southern California coast address the climate variability and timing of the penultimate deglaciation. Santa Barbara Basin sediments accumulated extremely rapidly providing climate records at unprecedented resolution for this time interval. They indicate that immediately before the penultimate deglaciation there was a brief (~2,000 year) warm interval, or interstadial, like those that occurred during the last glacial episode. There was also a brief climate reversal back to cool conditions during the deglaciation similar to the one that occurred during the last deglaciation known as the Younger Dryas. Although the marine cores cannot be dated directly, the brief climate events immediately before and during the deglaciation are also apparent in a climate record generated from a Chinese stalagmite. Because the climate events in the stalagmite can be accurately dated using the thorium-230 technique, these ages can be assigned to the events in the marine cores. This new chronology of the penultimate deglaciation suggests that the orbital theory of the ice ages is in fact correct. The complicated history of climate change during the deglaciation may account for the records of early deglaciation.

Mountain belts such as the western North American Cordillera and the Andes form some of the most impressive topographic features on Earth’s surface. Long recognized as having their origin in the interaction of Earth’s tectonic plates, hence their common reference as "’mobile belts," a persistent problem has been "Why are mountain belts high?" Commonly thought to be the result of a thick, buoyant crust, it is now recognized that this is true for only a few mountain belts, such as the Himalayas. Surprisingly, most mountain belts have only a thin crust. In a new twist, Dr. Roy Hyndman and colleagues at the Geological Survey of Canada suggest that the high elevations usually are the result of anomalously high temperatures in the mantle. The mantle beneath these mountain belts is hydrated by water released from the sediments and crust that are underthrust by the process of subduction. The rock-weakening effect of the water allows vigorous thermal convection in the mantle, like a slowly boiling pot of soup. Heat is carried rapidly to shallow depths, resulting in localized high temperatures. The mantle heat has two important effects: first, it decreases the density of the crust and mantle and helps support the height of the mountain belts. Second, the heat weakens the crust and mantle, allowing them to deform easily, localizing faulting and most earthquakes to these belts. Even after the subduction and input of water stop, mountain belts can remain hot and weak for more than 300 m.y. This hypothesis, elegant in its simplicity, finds strong support in the distribution of heat along modern plate boundaries, which suggests that higher than normal heat from the mantle plays a first-order role in the formation of mountain belts.

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